This invention relates to a method for obtaining drain-cast hollow articles and an apparatus therefor. More particularly, this invention relates to a method for drain-casting a slip and an apparatus therefor to obtain substantially hollow molded articles which are then burned to produce substantially hollow ceramic or porcelain ware such as sanitary ware, art ware, other ceramic water vessels or tanks, and the like.
Conventional methods for drain-casting slips comprise, for example, filling the mold cavity of a divisible gypsum mold which has been dried with a slip, depositing the slip onto the surface of the gypsum mold by the action of gypsum in absorbing water from the slip, draining undeposited slip remaining in the mold by gravity, increasing the strength of the resulting molded article by having the moisture of the deposited slip absorbed into the gypsum mold, and then removing the mold by hand operations to obtain the molded article. Such conventional methods, however, produce only two or less molded articles in 8 hours due to the limited water-absorption capacity of a dried gypsum mold, and the used gypsum mold which has absorbed water needs drying for 6 to 18 hours before it can be reused. Moreover, the production capacity per working period is very low and also the use-life of such gypsum molds is short (about 80 cycles) due to the deterioration which occurs in prolonged drying. Thus, the production cost of such molded articles becomes high. As one of the approaches to shorten the time required for drying the mold, there is disclosed, in U.S. Pat. No. 3,156,751, a water-absorbing mold for casting a slip wherein a porous conduit such as cotton rope is embedded and the conduit is communicated with a duct outside of the mold. In this case, compressed air is applied to the porous conduits through the duct to drive absorbed water out of the used mold for drying. As far as is known, however, such a mold is not practical because of troublesome hand operations for fabricating and/or drying the mold.
On the other hand, a method for cast-molding a slip to produce a solid (not hollow) molded article is disclosed in U.S. Pat. No. 3,243,860, wherein a mold including at least two mold portions supported by perforate steel back-up members is repeatedly used. The apparatus to be used to conduct the solid-casting method comprises a mold including at least two mating mold sections each including a perforate steel back-up member and defining a shape for the article to be molded. U.S. No. 3,243,860 is absolutely silent as to the step of draining an undeposited slip, and it is actually impossible to drain the undeposited slip from the disclosed apparatus. More specifically, with reference to FIG. 1 of the U.S. patent, the mold cavity of the mold 10 can be filled with a pressurized slip since the air present in the mold cavity is purged through the joint surfaces between the mold sections 12 and 14, although the mold sections are clamped together. The gaps between the joint surfaces are then clogged by deposition of the slip thereon. Thus, it is actually impossible to drain an undeposited slip from the mold cavity owing to a vacuum action, because the gaps are clogged and air can not pass through the gaps. In the process of the U.S. patent, the pressurization effect of the slip is lowered to 1/5 or less toward the end of the process because only the slip in the core region of the molded article is present in the state of highly viscous liquid, and thus it takes a longer timer to cast-mold the article. Even after removing the mold, it is thus impossible to drain the viscous slip from the molded article. Moreover, according to that patent, it is very difficult to take the article thus molded out of the mold without damage or deformation. It is necessary to stand the molded article in the mold for 30 minutes or more before demolding to harden the molded article, and even so about 50% of the molded articles will be damaged when a large-size article such as sanitary ware is demolded.
In this connection, it is noted that the patent states that "mold times of substantially less than five minutes are found to be possible" (Col. 4, lines 51 and 52 of the U.S. Pat. No. 3,243,860). This passage, however, is understood by those skilled in the art to mean that the cast-molding itself of small-size solid articles such as dishes may be conducted by dehydration of a slip in a mold in 5 minutes or less, but the article is demolded by hand operations after hardening it in the mold for a considerably longer period of time. Thus, the method and apparatus of the U.S. patent can not be used successfully for obtaining a large-size or heavy molded article such as sanitary ware efficiently, especially because of the difficulty in removing the mold.